<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        {
        margin: 0;
        padding: 0;
        }
        .content{
            width: 100%;
            height: 100vh;
            background-color: aqua;
            display: flex;
            flex-direction: column;
        }
        /*.content>header{*/
        /*    height: 60px;*/
        /*    background-color: #cccccc;*/
        /*}*/
        .content>header,.content>footer{
            height: 60px;
            background-color: #cccccc;
        }
        .content>main{
            /*height: calc(100% - 120px);*/
            flex: 1;
            display: flex;
        }
        .content>main>.left{
            width: 300px;
            background-color: #ececec;
        }
        .content>main>.right{
            flex: 1;
            background-color: #e4e;
        }
    </style>
</head>
<body>
<div id = "a" class="content">
    <header></header>
    <main>
        <section class="left"></section>
        <section class="right"></section>
    </main>
    <footer></footer>
</div>
</body>
</html>